I'm designing an app where in one screen of the application users can input a single number (and nothing else) by use of a numeric keypad. The number may have a single decimal place.
Depending on the function corresponding to the input field (in other words: depending on the screen the user navigated to), it might be the case that the user can enter only negative numbers into the field.
In this case of solely negative number input, I'd like to ease the input for the user. As no positive values are possible on that particular screen, I have the hunch that users should not be required to always start their input with the minus sign key. Users would save this superflous key press if the input field would just always be pre-filled with a minus sign at the beginning.
Now I'm asking me how the numeric (software) keypad I'm displaying to the user should look and feel. As there is a dedicated place for the minus key in the numpad, which users may know from other input fields on other screens, I'm struggling to decide between the following thee options:
- Hide the minus sign softkey from the numeric keypad.
- Grey-out the minus sign softkey and make it unusable.
- Show the minus sign softkey and allow users to optionally use it as first input character. This way, users may reach their desired negative input value (e.g. "-20.5") on two ways, either by omitting the minus key (here: "20.5", minus is pre-filled by the app) or by explicitly entering the minus key (here: "-20.5"). After entry of the minus key or entry of the first digit, the minus key would be greyed-out.